Polyester film continues to find new applications as stand-alone film or as a component in more complicated systems and devices. Its ability to satisfy the requirements of new applications lies in the perpetual modification and improvement of its properties, usually achieved through changes to its chemical character or its physical microstructure. The latter is linked intimately to its process or manufacturing history.
In many high added-value applications, a key property of polyester film is its dimensional stability, aspects of which can be characterized in terms of a number of different parameters, including the coefficient of linear thermal expansion (CLTE), the coefficient of linear hygroscopic expansion (CLHE) and the irreversible strain memory or “thermal shrinkage”. The invention discussed hereinbelow focuses on the thermal shrinkage behaviour of polyester film and in particular on achieving ultra low shrinkage through a low-cost treatment process.
The most common type of commercial film made from polyester has always exhibited an irreversible dimensional change at elevated temperatures, that is a permanent shrinkage or (under certain circumstances) expansion. This feature is a legacy of the manufacturing technology, which usually involves a stretching or drawing stage in one or two directions to enhance other properties such as strength, stiffness and toughness. The microscopic cause of the dimensional instability is relatively well understood and is attributed to the extension and alignment of segments of the polymer molecules which reside in non crystallized regions of the polyester material. Upon heating above the glass transition of the film, the molecular chains within the amorphous fraction of the film acquire sufficient rotational, vibrational and translational mobility to retract towards a more random, equilibrium conformation. On the macroscopic level, the film contracts physically. When expansion is observed, it is usually in a direction perpendicular to a large shrinkage and reflects a Poisson effect.
The change in dimension as a result of heat can be a few percent, however this value can be increased or reduced by a number of factors. For example if the manufacturing process can enable a physical relaxation of the film by a few percent during production it can almost entirely remove residual shrinkage in that direction. However while normal TD relaxation or “toe-in” is easily performed, continuous film processes operate with a finite line tension. Consequently in the most common form of commercial film manufacturing technology, MD relaxation is only partly achieved and a residual MD shrinkage of the film will manifest upon reheating.
A solution to this is offered by the biaxial stenter technology, which has the ability to apply strain relaxation in both the TD and MD. However the technology represents a high capital investment and data indicate that finite tensions still exist, leading to albeit small residual shrinkage. Alternative routes to allow relaxation under conditions of controlled temperature and tension or speed have also been developed, as online and offline post treatments. These post stenter relaxation treatments also operate at reduced line tension, but still cannot eliminate entirely the thermal shrinkage of polyester film, measured in the process direction.
The ideal conditions for thermal relaxation, which will remove all residual shrinkage behaviour in polyester film must allow for unrestrained or stress-free relaxation of the web. This is confirmed by a disclosure in JP-A-62/149415 which claims the benefits of annealing polyester film under such conditions, although the procedure comprised simply annealing small sheets of biaxial film in an oven over a temperature range of 150 to 220° C. The limitation to this batch process is that the stabilized film could not supply any downstream “roll to roll” processing.
The current situation is therefore that high costs are incurred to achieve the ultimate improvement in thermal shrinkage and in practice a balance is often reached between the cost of manufacturing stabilized polyester film, and its properties. However recalling that continuous improvement is a prerequisite to access new applications and new markets, there is a clear requirement for a low cost process that produces polyester film having ultra low thermal shrinkage in both its process and transverse directions.
A further technology, which is often employed to provide dimensional stabilization in polyester film is roll annealing, as disclosed in, for instance U.S. Pat. No. 4,141,735 and U.S. Pat. No. 6,071,682. In this approach, a roll of polyester film is heated from ambient temperature to a second elevated temperature, held under these conditions for a period of time and finally cooled. Care must be taken over the physical quality of the reel, the thermal gradient established as a result of heating and cooling and any other sources that may generate local regions of stress in the roll during treatment. However, this procedure is only claimed for low temperature annealing. In particular, temperatures below the glass transition temperature (Tg) of the polyester film are specified and optimally between 15 and 20° C. below this Tg. The treatment is known as “below-Tg annealing” (BTA) and is designed to improve the specific property of core set curl (CSC) in polyester film when it is subsequently wound and retained in final use, around a spindle of small diameter. The BTA process is a low cost, post-manufacturing treatment. The treatment exploits a fundamental behaviour of polymers often described as enthalpy relaxation which results in densification of disordered molecular chains and increased resistance to CSC. The thermal treatment is reversible if the polymeric film is reheated above its Tg. These factors; the optimum temperature range below Tg, the fundamental molecular process and its reversibility, and the macroscopic property and application of interest, make the BTA technology distinctly different to thermal stabilisation aimed at improving shrinkage behaviour.
The object of the present invention is to provide a low shrinkage polyester film, optionally also exhibiting good optical properties, and a process for the production thereof.